The present invention relates to a method and apparatus for non-destructively testing elongated objects such as wire rope or cable, rods, pipes and the like for loss of metallic cross-section and localized or distributed defects on the surface or within the object.
In my U.S. Pat. No. 4,659,991, I disclose a magnetic inspection device for testing elongated objects for loss of metallic cross-section, such as frayed cables, and local defects, such as individual broken wires forming the object. The device includes two pole pieces of opposite polarity for inducing magnetic flux in a section of the elongated object and a flux detector between the poles for detecting the changes of flux that arise due to the sought-for defects. The entire device is enclosed in a housing which is moved along the object to be tested or the object is pulled through the housing. Magnetic flux is induced into each portion of the elongated object, and the detector then senses flux changes to establish the presence or absence of the defect and the type of defect. The housing, the pole pieces and the detector are split into two at a hinge plane containing a hinge at one side of the housing for installing the inspection device on and removing the device from the object.
While the prior art device described above is satisfactory for measuring elongated objects up to a given size, there are still larger objects that cannot be handled by this prior art design with a single split housing. In particular, it has been learned that poor inspection results arise when there is insufficient magnetic saturation of the elongated object, and the larger the object, the more difficult it becomes to achieve saturation. For example, there are many suspension bridge cables and cables for mooring off shore drilling wells and exploration vessels that exceed 31/2 (9 centimeters in diameter). Complete or nearly complete saturation of such cables, nonetheless, is required for testing. Magnetic testing of ferromagnetic cables without magnetic saturation or near-saturation leads to large inherent noise levels which correspondingly reduce the signal-to-noise ratios in the inspection equipment. Tests have clearly established that inspections at high saturation levels are far superior to those at low saturation levels.
Complete magnetic saturation of large steel cables is, however, quite difficult because it requires large, heavy magnets. At the present time both electromagnets and permanent magnets can be employed. Compared to permanent magnets, however, electromagnets are not desirable since they require a steel core and copper windings which makes them very heavy and bulky. In addition a high power current source is needed to energize the electromagnets. The high current sources lead to substantial dissipated energy which in turn requires heat sinking and even further weight increases. Therefore the size and weight of electromagnets are much greater than equivalent permanent magnets.
Permanent magnets, although they are lighter than equivalent electromagnets, cannot be turned on and off. Consequently, when employed in large quantity and in sizes needed to fully saturate large cables, the magnets create strong magnetic attraction and repulsion forces between the magnets, the various magnetic components and the cable, and such forces lead to significant handling difficulties. Between the weight of the magnetic assemblies and the magnetic forces, the use of permanent magnets, though far superior to electromagnets, can be impractical for large cables.
It is accordingly an object of the present invention to provide both a practical magnetic inspection device and method for magnetically inspecting elongated objects which can be used to non-destructively test large elongated objects for loss of metallic cross-section and local discontinuities.